The present invention relates to syringe and, more specifically, to a safety syringe, which keeps the needle holder against vibration and prevents disconnection of the needle holder from the barrel during normal use and, enables the needle cannula with the body of the needle holder to be moved backwards with the plunger and received inside the barrel after the service of the safety syringe.
FIGS. 1 and 2 show a safety syringe according to the prior art. This structure of safety syringe comprises a barrel 10, a needle holder 20 holding a needle cannula 21, and a plunger 26. The barrel comprises a plurality of inside annular flanges 11 and 12 disposed around the inside wall of the front end thereof and defining two parallel inside annular grooves 13. The needle holder comprises a peripheral positioning flange 22 and two symmetrical rear hooks 23, and is mounted with a seal ring 24. The peripheral positioning flange 23 and the seal ring 24 are respectively engaged into the inside annular grooves 13 of the barrel 10, keeping the needle holder 20 secured to the front end of the barrel 10. The plunger 26 has a front end peripherally mounted with seal ring 25, and a front engagement block 27 forwardly axially extended from the front end. The front engagement block 27 has a peripheral groove 28. Before the use the safety syringe, the engagement block 27 does not touch the needle holder 20 (see FIG. 2A). After the service of the safety syringe, the engagement block 27 is forced into engagement with the rear hooks 23 of the needle holder 20, and then the plunger 26 is pulled backwards to carry the needle holder 20 and the needle cannula 21 backwards to the inside of the barrel 10. This structure of safety syringe is still not satisfactory in function. The drawbacks of this structure of safety syringe are numerous and outlined hereinafter.
1. The positioning of the needle holder 20 in the front end of the barrel is achieved by means of engaging the peripheral positioning flange 22 into one inside annular groove 13 of the barrel 10. Due to tooling technical problem. the protruding distance of the peripheral positioning flange 22 from the peripheral wall of the needle holder 20 is limited. In case an excessive forward pressure is imparted to the needle holder 20 during injection, the needle holder 20 will be forced out of the barrel 10.
2. Because the needle holder 20 has only two rear hooks 23 hooked on the engagement block 27 at two sides, the engagement between the engagement block 27 and the needle holder 20 is unstable. and the plunger 26 may slip from the needle holder 20 when pulled backwards.
3. Because the resisting force between the barrel 10 and the needle holder 20 is strong at the initial stage when pulling the plunger 26 backwards to carry the needle holder 20 back to the inside of the barrel 10, a rod-like tool or the like may be needed to push the needle holder 20 backwards. When forcing the needle holder 20 backwards with a tool or the like, the fingers may be injured by the needle cannula 21 accidentally.
4. When moving the plunger 26 forwards to squeeze medicine out of the barrel 10 through the needle cannula 21 into the patient""s body, the engagement block 27 is automatically forced into engagement with the rear hooks 23 of the needle holder 20, thus the safety syringe cannot be used again. In case the patient has to take two injections, a new safety syringe must be used.
The present invention has been accomplished to provide a safety syringe, which eliminates the aforesaid drawbacks. It is one object of the present invention to provide a safety syringe, which prevents vibration of the needle holder during the application of the safety syringe. It is another object of the present invention to provide a safety syringe, which enables the needle cannula with the body of the needle holder to be easily separated from the mounting unit of the needle holder so that the needle cannula and the body of the needle holder can be moved backwards with the plunger and received inside the barrel after the service of the safety syringe. According to one aspect of the present invention, the needle holder of the safety syringe has a mounting unit fixedly fastened to the front end of the barrel, a body holding a needle cannula outside the barrel, a breakable slotted connecting portion connected between the mounting unit and the body, and a rear engagement block. The plunger of the safety syringe has a front tubular retaining member adapted to engage the rear engagement block for enabling the needle cannula with the body of the needle holder to be separated from the mounting unit and moved backwards with the plunger to the inside of the barrel after the service of the safety syringe. According to another aspect of the present invention, the tubular retaining member of the plunger comprises a radially expansible front lip shaped like a tapered cylinder, the front lip having a thin bottom connecting portion connected to the retaining member. The front lip stops the tubular retaining member from engaging the rear engagement block of the needle holder unless the user gives a high forward pressure to the plunger against the rear engagement block of the needle holder. Thus, the safety syringe can be used twice for the same patient.